1. Field of the Invention
The present invention relates to a method of assessing the shear fatigue property of a rolling contact metal material and a method of and an apparatus for estimating the fatigue limit maximum contact pressure and, more specifically, to a method of and an apparatus for quickly assessing the shear fatigue property of a high strength metal material for rolling bearings such as, for example, a bearing steel.
2. Description of Related Art
For the assessment of the shear fatigue property, such testing machines as hydraulic servo type torsional fatigue testing machine and Schenk type torsional fatigue testing machine have been utilized, but the load frequency is limited to about 10 Hz at maximum in the case of the hydraulic servo type torsional fatigue testing machine and about 30 Hz at maximum in the case of the Schenk type torsional fatigue testing machine. Accordingly, where the fatigue limit is desired to be determined, assessment of the shear fatigue property to the ultra long life regime requires a substantial amount of time. High carbon chromium bearing steel, identified by JIS-SUJ2 according to the Japanese Industrial Standards, is currently most often used for a high strength metal material for rolling bearings and it is of a kind which is, after having been heated under the reducing atmosphere at a temperature (about 850° C.) higher than the AI transformation point, quenched and is then tempered at a relatively low temperature (about 180° C.) with its hardness being about 750 HV.
In the case of the rolling bearing, the subsurface-originating flaking failure that occur, when the lifetime is expired, under a favorable lubricating condition is considered attributable to the occurrence of and subsequent extension cracking resulting from repetition of orthogonal shear stresses (completely reversed stress) that exhibits the highest amplitude within the subsurface. In the case of the tension and compression fatigue test (axial load fatigue test, rotating bending fatigue test), it is institutive to deem the fatigue strength of 107 times as a fatigue limit. In contrast thereto, in the case of the rolling bearing, no subsurface-originating flaking failure occurs under the number of loadings of about 107 times even though a fairly high load is applied, if such rolling bearing is used under a good lubricating condition. While the torsional fatigue test is available as a test for inducing a fatigue fracture with shear stresses, the load frequency employed in the hydraulic servo type torsional fatigue test is 10 Hz at most and at least three years is required to reach the number of, for example, 109 loadings. For this reason, it is really impossible to determine the shear fatigue property up until the ultra long life regime.
Instead thereof, in view of the notion that the non-metallic inclusion, which appears to constitute a source of stress concentration as it is necessarily contained in steel and is structurally discontinuous, tends to constitute source of the subsurface-originating flaking failure, a technique of estimating the maximum size of the non-metallic inclusion, contained in an arbitrary volume, by means of an extreme value statistic analysis has been devised, and the method, in which the maximum size of the non-metallic inclusion is used as the quality index of the steel. In this respect, see the patent documents 1 to 4 listed below.